I have found an answer myself. I am posting it in case anyone is interested.

http://technet.microsoft.com/en-us/library/cc917680.aspx



>If you have mirroring set up, with the witness at the same location as the Mirror server (not the primary), and that site losses its external connectivity, the mirror and witness will both lose connection with the primary SQL server, and the mirror will assume primary status, even though the primary SQL Server is still online to the rest of the world (or its target consumers).
>
>If the witness is set up with the primary SQL Server, and both are taken out (by fire, etc.), then the witness will not automatically rollover.
>
>It would seem that a 3rd site is needed to perform this reasonably.
>
>I would appreciate any input on this matter from anyone who is or has used mirroring in a production environment.
>
>Your input is greatly appreciated.